Hand propelled vehicles for operation by a seated person commonly employ large diameter rear wheels with circular hand rails attached thereto for propulsion and steering. These have certain disadvantages. The wheel must be large to provide sufficient leverage, but the large wheel makes transport and storage awkward. The position of the arms during the power stroke prevents optimum use of the muscles, and requires an undesirable wrist twisting motion. Furthermore, while the hand is returning from the power stroke, the wheel on that side is not powered, wasting the energy of that motion, and it is uncontrolled. It may roll backward or turn, forcing the user to synchronize the power strokes bilaterally even though this may be uncomfortable or incompatible with a particular disability.
Hand crank drives are well known in the prior art in patents issued as far back as 1900 (Golata--U.S. Pat. No. 646,069) and as recent as 1985 (Tosti--U.S. Pat. No. 4,506,901). They use chain and sprocket crank drive that can provide continuous power. Dumont, U.S. Pat. No. 4,274,651 introduced a means for disengaging the crank arm by an outward motion so that the wheel can turn free of the crank arm for coasting and the cranks can be engaged at angles to one another that are most comfortable.
However the drive and clutch mechanism taught fail to satisfy the unique requirements of the wheelchair user. Dumont's clutch requires moving the handle in an outward arc. There may not be enough width clearance to perform this maneuver. It requires use of muscles not ordinarily developed by the user. Furthermore, the clutching mechanism abruptly engages the crank to the moving wheel, exerting a sudden load on the arms. Furthermore the braking actions taught by all the prior art wheelchair drives put all of the kinetic energy of motion onto the arms, which may not have normal strength.
Drive Structures that minimize sudden and excessive forces on the arms and optimize operation for the constraints imposed by physiologic limitations of a seated user will finally make hand crank drives practicable almost a century after their introduction. It is desirable, therefor, to provide an improved hand crank drive for wheeled vehicles that will overcome the many disadvantages of the prior art.